No easy answer here, unfortunately. That part, which is called a Head Block, appears to be custom made for that instrument. If that bass is more than 30 years old, I assume that it was made by a small boutique builder. And they made that head block for the bass, fitting it to that neck. It's not something that was mass produced and available anywhere as a replacement part.
I also don't see an easy way to repair it. The threads are stripped and it would be difficult to repair them . Possibly some steel threaded inserts?
The best permanent solution is to find someone with a small machine shop and have them make up a new custom replacement. Not a real difficult job. Drilled and milled from a block of brass, then shaped and polished to fit the neck. While they are at it, a few changes to the design to make it stronger and better.
I recommend getting rid of those slots. A bad design idea. The strings should just go through holes in the head block, and be clamped by the setscrews. Also, I would extend the thickness of the block a bit and put in two setscrews per string. The trick is to drill a small pocket under the outer setscrew of each pair. That way, tightening down the two setscrews puts a small Z-kink in the end of the string, which greatly improves the clamping. I've done that on a few headless bass head blocks that I've made in the past, and they worked very reliably.
